Photocopying method



United States Patent 0 3,380,823 PHOTOCQPYING M TEHPD Robert Murray Gold, Brooklyn, NFL, assignor to Itek Corporation, Lexington, lass, a corporation of Delaware No Drawing. Continuation-impart of abandoned application Ser. No. 137,476, Sept. 7, 1961. This application June 2%, 1966, Ser. No. 558,630

16 Claims. Zl. 96-47) ABSTRACT OF THE DESCLQSURE This invention relates to a process for recording an image pattern of activating radiation comprising exposing to an image pattern of activating radiation a copy medium comprising at least one photosensitive semiconductor pigment such as titanium dioxide which becomes reversibly activated upon exposure to activating radiation and thereby capable of causing chemical reactions at portions of this medium corresponding to the image pattern of activating radiation and then applying to at least reversibly activated portions of this medium an image-producing agent which undergoes a chemical reaction and which forms irreversible images solely on contact with at least activated portions of the copy medium. The image-producing agent comprises, for example, an aqueous solution of silver nitrate and a suitable reducing agent therefor.

This application is a continuation-in-part of US. patent application Ser. No. 137,476 filed Sept. 7, 1961, and now abandoned.

The present invention relates to a process of photoimaging and more particularly to a method of preparing high-contrast images employing semiconductor materials.

Various types of photographic and reproduction materials and methods are known, a number of which employ semi-conductor and photo-conductive materials. One popular imaging method is the electrophotographic process which employs a coating of photoconductive pigments on a carrier sheet material of electroconductive properties. Applying to the photoconductive layer a uniform electrostatic charge and exposing the layer to a light image results in an imagewise distribution of differentially charged areas on the sheet material. The charge distribution is then developed to a visible image by application of a suitable finely divided toner material which adheres selectively to the electrostatically charged portions of the coated base sheet. Although the electrophotographic method has found wide use, it remains particularly expensive in view of the equipment requirements and the preparation of highly specialized base sheet materials.

A method related to electrophotography is the photoconductographic process which employs the photoconductive activity associated with semi-conductor pigment materials similar to those used in electrophotography and by photo-imaging creates an imagewise distribution of current-conducting pathways through the copy material. Such areas of selective conductivity are employed, with current application and developer compositions including electroplating materials, to produce visible images corresponding to the imagewise distribution of the conductive pattern. As with the electrophotographic process, the base sheet materials are generally required to be conductive and copying equipment is expensive and particularly complicated.

According to the present invention, a copying method is provided which makes use of the electrophotographic properties of semi-conductor pigment materials, but which may be accomplished inexpensively without the use of Patented Apr. 30, 1968 "ice complicated equipment and which further makes use of less expensive, non-conducting base sheet materials.

Photocopying processes have previously been described, for example by Levinos in U.S. Patent 3,052,541, which employ a combination of semi-conductor pigments and solutions of reducible ions. lmagewise light exposure of the pigment/ion combination results in the reduction of the ion to form an immediately visible image. A particular disadvantage of such photo-imaging methods has been the requirement that the photo-reactive composition be in a fluid condition or at least under the influence of a fluid medium during the moment of photo exposure. The copy device and apparatus requirements therefore have included the provision of reservoirs of sensitizing solutions with the usual resulting difiiculties occasioned by fluid handling and storage.

The present invention makes use of the discovery that semiconductor pigments normally employed in the aforementioned photonconductographic, electrophotographic, and photochemical copying methods possess the ability to retain a latent image upon exposure to light and that the latent image will retain for a substantial length of time suhicicnt energy to initiate an electrochemical reaction by which a visible image may be formed. Apparently the energy absorbed at the semi-conductor band gap during exposure to actinic radiation will be retained within the photoconductor over extended periods of time and can be thereafter transferred as the energy of activation for image-forming oxidation/reduction reactions. This latent image phenomenon has heretofore not been applied in a photocopying or photo-imaging process, since it was apparently believed that the photolytic properties of semi conductor pigment materials could be employed only where conditions of actual current flow, such as by electrostatic charge dissipation or electromechanical plating reactions, were involved.

The present invention therefore provides a method of photo-imaging a non-conductive base sheet material and further provides a photo-imaging method which eliminates the requirement for fluid or electroconductive media during periods of photo exposure.

The process according to the present invention consists essentially of providing a supporting sheet material such as paper or plastic film with a coating of a semi-conductor material such as the zinc oxide or titanium dioxide normally employed in previously described electrophotographic compositions. The semi-conductor surface is generally prepared in the usual manner associated with the coating art and will preferably include up to about 25% of a relatively inert insulating binder resin material. The semi-conductor coating is exposed imagewise in usual contact or projection imaging equipment and as a result of such exposure a latent image is formed corresponding to the light-struck areas of the coated surface. The latent image is retained for a substantial period of time, in some instances a period of a few days, and development may be accomplished any time dur'mg that period at the leisure of the operator. The development of the latent image is simply performed by contacting the exposed area of the semi-conductor coating with a composition containing a readily reducible material, for example, a solution of a noble metal salt. Upon contact of the developing agent the stored energy corresponding to the distribution of photinic radiation causes an immediate reduction of the developer material with resulting formation of a visible copy of the projected or contact print master image.

The amount of developer material employed may vary with the covering power of the particular developing substance and selection of developer concentrations are well within the purview of the skilled technician. It has been additionally found that combinations of reducible and oxidizable developer materials will often result in effective, dense image formation. Lower concentrations of reducible materials may thus be employed, and, in combination with subsequently applied oxidizable color-forming materials in low concentrations, will, by consequent oxidation, effect dense color formation. Such oxidiza ble color formers include secondary polyamines such as para-phenylenediamine, dimethyl aniline, and orthotolidine derivatives.

The present invention will be more apparent with reference to the following examples setting forth some preferred embodiments of the photo-imaging process:

EXAMPLE 1 A coating composition of about 75 parts of titanium dioxide dispersed in a resinous solution binder containing about 25 parts of a styrene butadiene polymer is applied in a uniform coating on a film of polyethylene terephthalate. Prime coating or subbing of the base film in a manner known in the coating art may be accomplished prior to coating to improve the adherence of the pigmented coating. The dried, coated material is exposed to a light image projected from a master transparency. The exposed sheet may be stored for a period of about two days without loss of the latent image formed. Image development is effected by contacting the coated surface of the sheet with a solution of silver nitrate for a time sufficient, depending upon the solution concentration, to produce an observable image corresponding to the light-struck areas of the semi-conductor coating. A contact print may be similarly prepared with equally good results.

EXAMPLE 2 A base sheet is coated with a composition including a mixture of zinc oxide, titanium dioxide, lead chromate and lead molybdate in a polymeric resinous binder solution. The dried sheet material is exposed by normal contact methods with light of a broad spectrum band. After exposure the coated sheet is contacted with a solution of silver nitrate and a copy image is formed directly. The image is readily fixed by washing the developer solution from the surface and drying. The mixture of the semi-conductor pigments provides a Wider range of spectral sensitivity. A preferred range may be provided by varying the proportions of the ultraviolet sensitive zinc oxide and the other pigments which are responsive to longer wave lengths, the lead chromate and lead molybdate being more responsive to the longer Wave lengths of light.

EXAMPLE 3 A paper base is coated with a titanium dioxide resin binder composition and is exposed to light through a graphic master, thus forming a latent image. The latent image may be stored for about two days. The image is developed by contacting the pigment-coated surface with a 3% solution of silver nitrate in alcohol, the excess solution being wiped off. A particularly dense image is subsequently obtained by contacting the silver nitrate-moistened surface with a 1% alcohol solution of N,N'-dinaphthyl-para-phenylene-diamine. The use of the oxidizable polyamine color developer provides a greater image intensity, compared with silver nitrate developing solutions alone, when the exposure is limited by time or light source. Similarly good results are obtained with polyamine developer solutions containing 4,4',4"-methylidyne-tris (N,N) dimethyl-aniline, or orthotolidine.

I EXAMPLE 4 A paper base sheet coated with a composition of zinc oxide and a polymeric resin binder is exposed to near ultraviolet light through a master transparency and the latent image thus formed is thereafter developed to a dense, high contrast image by contacting the coated surface of the sheet with a solution comprising citric acid and silver nitrate. A visible image of desirable density of the developed image is provided by a developer solution comprising hydroquinone and silver nitrate. Similar results are obtained by substituting titanium dioxide for the zinc oxide pigment of the coating composition. Exposure light of longer wave length may be advantageously employed with the latter composition.

EXAMPLE 5 A paper-based zinc oxide/binder resin coating exposed to a moving light beam thus forming a latent image. The image is developed subsequent to exposure by contacting the coated surface of the sheet with a solution of silver nitrate. As soon as sufficient density of image is obtained from the contact with the solution of silver nitrate, the image is fixed by simply drying the developer solution at the sheet surface. Good permanent fixation is obtained also by contacting the developed-image surface with a solution of thiourea.

The above examples have been presented for the purpose of illustration and should not be taken to limit the scope of the present invention. It will be apparent that the described examples are capable of many variatitons and modifications which are likewise to be included within the scope of the present invention as set forth in the appended claims.

What is claimed is:

1. A process for recording an image pattern of activating radiation comprising exposing to an image pattern of activating radiation a copy medium comprising at least one semiconductor pigment which becomes reversibly activated upon exposure to activating radiation and thereby capable of causing chemical reactions at portions of said medium corresponding to said image pattern of activating radiation and then applying to at least reversibly activated portions of said medium an image-producing agent which undergoes an oxidation-reduction type and which forms irreversible images solely on contact with at least activated portions of the copy medium.

2. A process for recording an image pattern of activating radiation in the absence of an externally-applied electric field, comprising exposing to an image pattern of activating radiation a copy medium comprising at least one semiconductor pigment which becomes reversibly activated upon exposure to activating radiation and thereby capable of causing chemical reactions at portions of said medium corresponding to said image pattern of activating radiation and then applying to at least reversible activated portions of said medium an image-producing agent which undergoes an image-forming oxidation/reduction reaction to form visible images when contacted with at least activated portions of the copy medium.

3. A process for recording an image pattern of activating radiation in the absence of an externally-applied electric field, comprising exposing to activating radiation a copy medium comprising at least one metal containing photoconductor which becomes reversibly activated upon exposure to activating radiation and thereby capable of causing chemical reactions at portions of said medium corresponding to said image pattern of activating radiation, thereby reversibly activating the radiation-struck portions of said copy medium and then applying to at least reversibly activated portions of said medium a readily reducible developing agent reacting on contact at said reversibly activated portions to form reaction products corresponding to said image pattern, said photoconductor remaining essentially unchanged chemically after contact with said developing agent.

4. A process as in claim 3 wherein said metal containing photoconductor is an inorganic compound.

5. A process as in claim 3 wherein said metal containing photoconductor is present in the :form of finelydivided particles insoluble in said liquid redox system and dispersed in a resinous binder on a suitable backing.

6. A process as in claim 3 wherein the metal containing photoconductor is a semiconductive metal oxide or metal sulfide and wherein the developing agent comprises a metal ion containing solution.

7. A process as in claim 6 wherein the reducible metal ion is obtained from a silver nitrate solution and wherein the silver nitrate solution and the reducing agent therefore are applied to the exposed copy medium in separate steps.

8. A process as in claim 3 wherein the developing agent comprises, (1) a solution comprising readily reducible metal ions, and (2) a solution comprising a non-oxidized readily-oxidizable color former, said color former being colorless in the non-oxidized form and visibly colored in the oxidized form, whereby said ions are reduced upon contact with at least reversibly activated portions of said medium and said color former is correspondingly oxidized to said colored form. V

9. A process as in claim 8 wherein said metal ions are silver ions, and said color former is a secondary polyamine selected from the group consisting of N,.N-dinaphthyl paraphenylenediamine, 4,4',4" methylidyne tris (N,N')dimethyl-aniline, and orthotolidine.

10. A process as in claim 8 wherein the photoconductor compound is selected from at least one member of the group consisting of zinc oxide, titanium dioxide, lead chromate and lead molybdate.

11. A process as in claim 10 wherein the photoconductor is in the form of finely-divided particles dispersed in a resinous binder on a suitable substrate.

12. A process as in claim 11 wherein said photoconductor compound is titanium dioxide.

13. A process as in claim 12 wherein the reducible metal ion is obtained from a silver nitrate solution, and wherein the silver nitrate solution and the reducing agent therefor are applied to the exposed copy medium in separate steps.

14. The method of photo-imaging which comprises:

(a) providing a surface of a support with a coating of a semi-conductor pigment which becomes reversibly activated upon exposure to activating light;

(b) exposing the semi-conductor surface to a light pattern thereby producing a latent image upon said surface; and

(c) applying a readily-reducible developing agent to said exposed surface, thereby rendering said latent image visible.

15. A process for rcording an image pattern of activating radiation in the absence of an extrnally-applied electric field, comprising exposing to an image pattern of activating means a copy medium comprising a photoconductor compound selected from at least one of the group consisting of metal oxides, metal sulfides, metal chromates and metal molybdates which become reversibly activated upon exposure to said activating means and thereby capable of causing chemical reactions at portions of said medium corresponding to said image pattern of activating means and then applying to at least reversibly activated portions of said medium a developing agent comprising (1) a reducible metal ion containing solution, and (2) a reducing agent for said reducible metal ion.

16. A process of producing a visible image, in the absence of externally-applied electric fields, in a copy medium comprising a photosensitive titanium dioxide, which process comprises exposing said medium to an image pattern of activating radiation, thereby activating said titanium dioxide to render it capable of causing chemical reaction at portions of said medium corresponding to said image pattern of activating radiation, and then developing activated portions of said medium by contacting at least said activated portions With a readily reducible developing agent reacting on contact to precipitate free metal on said medium at said activated portions thereof and to precipitate further free metal on those portions of said medium where free metal is already present, thereby forming reaction products defining a visible image corresponding to said image pattern.

References Cited UNITED STATES PATENTS pp. 763768 (December 1959).

Chem. Abstracts 7062 f, Korsunouskii, Problemy Kinetiki i Kataliza, Akak. Nauk S.S.S.R. (1960).

NORMAN G. TORCHIN, Primary Examiner.

J. TRAVIS BROWN, Examiner.

C. E. VAN HORN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,380,823 April 30, 1968 Robert Murray Gold It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 34, after "type" insert -reaction line 45, "reversible" should read reversibly Column 5, line 444 "rcording" should read recording line 45, "extrnally" should read externally Signed and sealed this 16th day of September 1969.

(SEAL) attest;

ldwgrd M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Lttesting Officer Commissioner of Patents 

